1. Field of the Invention
This invention generally relates to a magnetic recording reproducer for reproducing television signals recorded on a magnetic tape and more particularly to an automatic tracking system of the magnetic recording reproducer.
2. Description of the Related Art
When television signals recorded on a magnetic tape by a video tape recorder (VTR) are reproduced or played back by another VTR, the picture quality of the reproduced picture is sometimes poorer than that obtained in case where the television signals are played back by the same VTR used for recording thereof on the tape. This is because of a tracking error occurring due to the fact that these two VTRs are not completely identical with each other in mechanical and electrical precision and thus, as shown in FIG. 5, the video head of the VTR used for the playback of the television signals does not always precisely trace the central axis or line, which is indicated by a one-dot chain line in this figure, of a recorded diagonal track on the video tape, that is, there may occur a difference between the path of the video head and the central line of the track. At that time, an operator or user must to adjust a tracking control switch of the VTR to the position, at which the best picture quality can be obtained, observing the screen of a display device connected to the VTR or a Hi-Fi track level meter of the VTR.
One of the recent advances in technology is the use of a magnetic recording reproducer provided with an automatic tracking system which can automatically perform the entire tracking control to resolve the above described problem.
With such a magnetic recording reproducer having an automatic tracking system for automatically obtaining the optimum tracking, it is necessary to detect the level of the signals to be detected. For example, video FM signals are processed and audio frequency modulated (FM) signals by integrating and averaging outputs of a detecting circuit, which is used for detecting these signals as an interface circuit, corresponding to one frame. Further, the automatic tracking system is adapted to search or detect the optimum tracking position on the basis of the detected level of the signal.
Conventionally, when the waveform of a predetermined section of a signal is integrated and averaged, the result thereof is obtained by first sampling the inputted waveform at sampling intervals, each of which is sufficiently short in comparison with an integral interval, performing analog-to-digital (AD) conversion of the sampled data and further effecting integration processing.
FIG. 6 shows a conventional level detecting circuit of such a type, in which the video and audio FM signals recorded on the magnetic tape are detected by the head and are further rectified by a detecting circuit 2 through an amplifier 1 and as a result, are converted into a direct current (DC) signal having a constant level. The high-frequency components are eliminated through a low-pass filter (LPF) 3. Furthermore, the analog values indicated by the thus obtained DC signal are converted by an AD converter 4 and are added by an adder 5 so as to obtain an integrated value as a result of the integral of the FM signals within a predetermined time interval, for example, one frame period.
Thus, the variation in level occurring due to the misalignment of the linearities of the magnetic track and the path of the head is eliminated and averaged by using the integrated value obtained by such a level detecting circuit, and the automatic tracking system is adapted to detect the tracking position of the head, at which the integral of the signal of a frame reaches a maximum value thereof, on the basis of the result of the level detection.
The level detecting circuit, however, requires sampling many data in the integrating interval with respect to time for calculation of the integral of the signal and further performing the integration by using the sampled data, as described above. Furthermore, to obtain accurate samples or data, it is necessary to insert a low-pass filter 3 having sharp cut-off characteristics just prior to an AD converter 4. Therefore, in case where the processing such as the calculation of the integral of the signal is carried out by a central processing unit (CPU), a load on the CPU becomes considerably large. Thus, a CPU dedicated to such processing is needed. This results in a complex configuration of the circuit and a high manufacturing cost.
Further, if an appropriate integral time constant is selected in such a manner to obtain a satisfactory mean value of the level, the selected time constant becomes very large. In case of the application of such a conventional technique to the automatic tracking system, although it is preferable to change the tracking position at a high speed in order to immediately obtain the integral of the FM signal, which is to be reproduced over a corresponding predetermined intergrating interval, for instance, one frame, actual response to the change of the tracking position is slow, and thus an accurate value of the integral thereof cannot be obtained.
For example the cut-off frequency of the low-pass filter 3 is fc; and the sampling frequency of the AD converter 4 be fs. Further, to reduce aliasing errors, a low-pass filter is required having sharp cut-off characteristics, of which the cut-off frequency fc is one-half of the sampling frequency fs. However, if the sampling frequency fc is made higher in order to get real time information, it is required that a great number of AD conversions of data obtained within a predetermined intergrating interval is performed and further at each of the AD conversions an addition of the converted data is effected. As a result, the processing time or executing time of executing programs becomes longer and thus, the load on the CPU becomes large especially in case of effecting parallel processing. On the other hand, when the cut-off frequency of the low-pass filter is reduced to decrease the number of times of effecting the AD conversions, there is a problem that the quick control of the tracking cannot be effected because of the delay occurring between the outputting of the integral of the reproduced FM signal and the change of the reproduced FM signal.